body {
    --xx: 500px;
}

.back {
    position: fixed;
    width: 100%;
    height: 100%;
    background: url(../img/Christmas_Trees.png) no-repeat;
    background-size: 100%;
    z-index: 1;
}

.back::after {
    width: 100%;
    height: 100%;
    content: "";
    position: absolute;
    background: inherit;
    top: 0;
    left: 0;
    filter: blur(10px);
    z-index: 2;
}

.main {
    background-color: rgba(97,97,97,0.7);
    width: var(--xx);
    height: var(--xx);
    border-radius: 10px;
    position: absolute;
    top: calc(50% - var(--xx)/2);
    left: calc(50% - var(--xx)/2);
    z-index: 10;
    padding: 10px;
    padding-top: 30px;
    font-size: 17.5px;
}